1. Field of the Invention
This application relates to the field of fluid dynamics and more particularly to the field of propellers or windmills.
2. Description of Related Art
Propellers are able to propel objects by exploiting the principle of conservation of total linear momentum. Propeller blades accelerate fluid so that the speed of the fluid entering the propeller is lower than the speed of the fluid exiting. The propeller, and whatever is attached thereto, must therefore move in the forward direction to ensure the conservation of total linear momentum. Because of this difference in fluid speeds, the concentration of streamlines at the rear of the propeller blades is higher than at the front. Or, what is the same, A1&gt;A2, where A1 and A2 are the cross-sectional areas, perpendicular to the streamlines, of a tube of flow fore and aft of the propeller. The larger the ratio A1/A2, the more energy can be put into the airstream.
In conventional blades, however, blade tip vortices and other inefficiencies are introduced as the fluid near the blade tips escapes from the rear to the front. These tip losses result in a diminution of A1/A2. I.e., these tip losses reduce the concentration of the streamlines at the rear of the propeller and increase the concentration at the front, leading to undesirable loss of thrust and efficiency.
Another problem shared by conventional propellers is that the optimum blade size needed to produce the required thrust is often too large to allow the propeller to turn. For example, the size of a propeller producing the required thrust for an airplane may be too large to clear the tarmac while rotating. Or, in another example, the optimum blade size producing the desired thrust for a boat may be such as to impede proper functioning of a boat propeller in shallow water. While truncating the blades would solve the aforementioned problem, this truncation may result in more tip loss and less efficiency. Also, as blade diameter increases, a gearbox ratio also increases, adding weight and reducing efficiency.